1. Field of the Invention
The present invention relates to a displacement information detecting apparatus and a doppler velocimeter apparatus for determining the velocity of a moving object (solid or fluid) without contacting the moving object or fluid. This invention is particularly suitable for a laser doppler velocimeter which determines the velocity of an object by detecting the frequency shift of a laser beam.
2. Description of the Related Art
Laser doppler velocimeters are widely used to precisely determine velocities of moving objects (solid or fluid) without contacting the moving objects or fluids. To determine the velocity of a moving object or fluid, a laser doppler velocimeter emits laser light to the solid or fluid object and detects a Doppler effect, in which the frequency of light scattered from the object shifts in proportion to the velocity of the object.
With reference to FIG. 1, a conventional laser doppler velocimeter will be described. Laser light emitted by a laser source 1 is formed into a bundle of parallel rays 3 by a collimator lens 2, split into two bundles 5a and 5b by a beam splitter 4, and reflected by mirrors 6a and 6b, respectively, so that the two ray bundles 5a and 5b are incident on an object 7 moving at a velocity V, at an incidence angle of 9. Light scattered from the object is condensed by a condenser lens 8, and detected by a photodetector 9. The frequencies of the light scattered from the two ray bundles shift by Doppler shifts of +f and -f which are proportional to the velocity V. The Doppler shift f is expressed by the following formula (1): EQU f=Vsin.theta./.lambda. (1)
where .lambda. is the wavelength of the laser light. The scattered rays whose frequencies have been Doppler-shifted by +f or -f interfere with each other so as to cause bright-dark cycles on the light receiving surface of the photodetector 9. The frequency F of the cycles is given by the following formula (2): EQU F=2f=2sin.theta./.lambda. (2)
Therefore, if the frequency F of the output signal from the photodetector 9 (hereinafter, referred to as the "Doppler frequency F") is determined, the velocity V of the moving object 7 can be obtained on the basis of formula (2).
As formula (2) indicates, the Doppler frequency F detected by the above-described laser doppler velocimeter is in inverse proportion to the wavelength .lambda. of the laser light. Therefore, this laser doppler velocimeter must employ a laser source which emits laser light having a constant wavelength. Gas lasers, such as He-Ne lasers, which are able to continuously oscillate and stabilize wavelengths, are widely used in laser doppler velocimeters. However, because the laser oscillators used for gas lasers are bulky and require a high-voltage supply, gas lasers are usually large and expensive. Laser diodes (semiconductor lasers), which are widely used in compact disc players, video disk players, optical fiber communication apparatuses or the like, are very small and can be easily driven, but the wavelength of laser diodes significantly vary depending on temperature.
FIG. 2 indicates an example of normal temperature dependency of a laser diode (adopted from Mitsubishi Semiconductor Handbook (1987), vol. "Optical Semiconductor Device") . During continuous changes, the wavelength varies at a rate of 0.05-0.06 nm/.degree. C., depending mainly on temperature-dependent changes in refractivity of the active layer of the laser diode. During discontinuous changes, which are called "longitudinal mode hopping", the wavelength discontinuously varies at a rate of 0.2-0.3 nm/.degree. C.
In a usual method for stabilizing the wavelength, a laser diode is maintained at a constant temperature. However, in this method, the temperature must be precisely controlled by using various temperature controlling members, such as a heater, a radiator and a temperature sensor, which are attached to the laser diode so as to have only small thermal resistances. Therefore, employment of this method increases the size and cost of the laser doppler velocimeter. Further, this method cannot completely eliminate wavelength instability caused by the above-mentioned longitudinal mode hopping.
To eliminate the above-described problems, Japanese Patent Application Laid-open No. 2-262064 proposes a laser doppler velocimeter employing a diffraction grating. In this velocimeter, laser light emitted from a laser strikes a diffraction grating, and the two ray bundles of the orders +n and -n (n=1, 2, 3, . . . ) of the diffracted light (excluding the ray bundle of zero order) are directed so as to strike a moving object or fluid at intersecting angles equal to the angle between the two bundles of rays. Then, the photodetector of the velocimeter detects light: scattered from the object or fluid.
FIG. 3 illustrates an example of diffraction where laser light I is incident on a transmission diffraction grating 10 having a grating pitch of d, perpendicularly to the direction t in which the grating lines are arranged. The diffraction angle .theta..sub.0 is given by the following formula: EQU sin.theta..sub.0 =m.lambda./d
where m is a diffraction order (0, 1, 2, . . . ), and .lambda. is the wavelength of the light. The ray bundles of the order in except the order zero are expressed by the following formula: EQU sin.theta..sub.0 =.+-.n.lambda./d (3)
where n is a natural number (1, 2, . . . ).
FIG. 4 illustrates the ray bundles of the order .+-.n which are reflected by mirrors 6a and 6b so as to strike an object 7 at incident angles equal to .theta..sub.0. Numeral 61 denotes a main body of this apparatus. By using the formulas (2) and (3), the Doppler frequency F detected by a photodetector 9 is obtained as: EQU F=2Vsind.theta..sub.0 /.lambda.=2nV/d (4)
Thus, the Doppler frequency F is independent from the wavelength .lambda. of the laser light I, in inverse proportion to the grating pitch d of the diffraction grating 10, and in proportion to the velocity V of the determination object 7. Since the grating pitch d can be made substantially constant, a Doppler frequency F detected by the photodetector 9, practically speaking, is simply in proportion to the velocity of the determination object 7. The diffraction grating 10 may be of a reflection type instead of a transmission type, achieving substantially the same effects.
The present inventors have proposed in published European Patent Application No. 0458274 a laser doppler velocimeter as described above having an optical system in which the incidence angle .theta. of laser light striking a moving solid or fluid object varies in accordance with changes in the wavelength of the laser light so that sin.theta./.lambda. remains substantially constant, the optical system includes a diffraction grating for diffracting the laser light so as to form diffracted ray bundles of the orders .+-.n (n=1, 2, 3, . . . ) and two lenses which have the same focal lengths and are arranged apart from each other by an interval twice the focal length F. As used in this specification, an interval between lenses or lens groups is defined as a distance between an image field-main plane (that is, the main plane closer to the image field) of a lens (or a lens group) closer to the object and the object-side main plane of a lens (or a lens group) closer to the image field, and an interval between a lens (or a lens group) and an object other than a lens, such as an object or an optical component other than lens, is defined as a distance between the object and the object-side main plane of the lens (or a lens group). The thus-constructed laser doppler velocimeter is able to prevent deviation of the two intersecting ray bundles despite a change in the wavelength of the laser light. Further, the laser doppler velocimeter achieves an increased working distance and, thereby, good operability by setting the interval between the diffraction grating and the neighboring lens to a distance which is shorter than the focal length.
FIG. 5 schematically illustrates essential portions of an optical system of a laser doppler velocimeter employing a diffraction grating. FIG. 5 shows an object 7 whose velocity is to be determined, and a ! laser doppler velocimeter 101 comprising: a laser diode 1, a collimator lens 2, a diffraction grating 10, and convex lenses 11L, 12L (illustrated as thin lenses in the figure) having a focal length of f, the foregoing members being arranged as shown in the figure. The distances a and b satisfy the formula, a+b=2f.
The laser diode 1 emits laser light having a wavelength of about 0.68 .mu.m, which is formed into a parallel ray bundle 3 having a diameter of 1.2 by the collimator lens 2. The parallel ray bundle 3 perpendicularly strikes the transmission diffraction grating having a grating pitch of 3.2 .mu.m. As a result, diffraction ray bundles of the orders .+-.1 go out therefrom at diffraction angles .theta.=12.degree.. The ray bundles 5a and 5b enter the convex lens 11L with the focal length f and come out as ray bundles 13a and 13b. Then, the bundles 13a and 13b enter the other convex lens 12L placed 2f apart from the convex lens 11L and come out as parallel ray bundles 14a and 14b, which strike the object 7 at incidence angles equal to the diffraction angle .theta.=12.degree. so as to form a light spot having a diameter of 1.2 mm on the surface of the object 7. Laser light scattered from the object 7 is effectively converged by a combination of the convex lens 12L and a condenser lens 8 onto the photoreceptor portion of a photodetector 9, which then detects light signals including a Doppler signal expressed by the following formula (5): EQU F=2V/d (5)
If the wavelength .lambda. laser light emitted by the laser diode 1 varies, the diffraction angle .theta. varies. However, similar to the velocimeter described above, such variation of the wavelength .lambda. or diffraction angle .theta. does not affect the Doppler signal F. This laser doppler velocimeter is able to fix the position of the spot formed on an object by the two ray bundles. If the position of the laser doppler velocimeter 101 relative to the object 7 is set as shown in FIG. 5, the spot formed on the object 7 by the two ray bundles 14a and 14b remains in the same position relative to the laser doppler velocimeter 101. Therefore, the two ray bundles 14a and 14b constantly coincide on the object 7 and never form two spots that deviate from each other.
Further, since a&lt;b, that is, the distance b is relatively long, a long working distance is achieved, thus substantially increasing freedom when installing the velocimeter.
To achieve high performance of the above-described laser doppler velocimeter, the interference fringes formed at the intersection of the two ray bundles must be constantly stable in any depth at which determination is performed, despite changes in the wavelength of the laser light.
Collimation (parallelization) of laser light will now be described. FIG. 6 shows an enlarged view of the intersection of the two ray bundles (encircled in FIG. 5). If the collimation of the laser light is poor due to, for example, aberration of the collimating optical system and, therefore, the ray bundles diverge to some extent, the interference fringes have different intervals at locations closer to and farther from the laser doppler velocimeter. Therefore, even if objects move through the ray bundle-intersecting space at the same velocity, the resulting velocity determinations vary depending on how far the course of a moving object through the space is from the laser doppler velocimeter. More specifically, the doppler frequency with respect to an object moving at a constant velocity varies depending on determination depths. The same problem is caused if the ray bundles converge.
The interference fringe interval p is written as: EQU p=.lambda./sin(.theta.n) (6)
where .theta.n is half the angle between the two ray bundles.
The wavelength .lambda. is written by using the diffraction angle .theta..sub.0 with respect to the diffraction grating having a grating pitch d as follows: EQU dsin(.theta..sub.0)=.lambda. (7)
If there is an angle deviation .theta.' from the diffraction angle .theta..sub.0 of each ray of the ray bundles, the angle deviation .theta.' being a parameter indicating an error in the collimation of the ray bundles, then .theta.n=.theta..sub.0 +.theta.' (.theta..sub.0 is the diffraction angle and becomes equal to the angle .theta.n if there is no angle deviation). Thus, the interference fringe interval can be expressed as follows: EQU p=.lambda.k/2sin(.theta..sub.0 +.theta.') (8)
If the ray bundles are perfectly collimated, that is, .theta.n=.theta..sub.0, then the interference fringes have a desirable interference fringe interval p, which can be written on the basis of the formulas (6) and (7) as follows: EQU p=d/2 (9)
If absolute precision of +0.2% is required, that is, a value p obtained on the basis of the formula (8) is allowed to deviate from a value p obtained on the basis of the formula (9) by .+-.0.2% or less, then the angle deviation .theta.' must be about 1 minute or less.